Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. In many wind turbines, the generator may be a wound rotor, three-phase, double-fed induction (asynchronous) generator (DFIG) that includes a generator stator magnetically coupled to a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. The associated electrical power can be transmitted to a main transformer that is typically connected to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
The rotor voltage of a DFIG is approximately proportional to the applied stator voltage, the slip frequency and the turns ratio of the DFIG. Further, the rotor voltage is also heavily affected by VAR production. Balancing the turns ratio and the current carrying capacity of the rotor circuit is an important design choice of the generator. For example, a high turns ratio reduces the current but increases the rotor circuit voltage that occurs with slip. In order to produce an economical turbine with a small power converter, the rotor turns ratio is selected to just accommodate a certain speed range, which minimizes the amount of current that must be used to produce rated power.
Based on historical sizing of DFIG machines, the typical turns ratio of a wind turbine produces a rated rotor voltage at sync speed +/−⅓ sync speed. For example, a 60 Hertz (Hz) machine will have a synchronous speed of 1200 rotations per minute (RPM). At ⅓ slip (i.e. 400 RPM slip), the rotor produces rated voltage, which corresponds to 800 RPM and 1600 RPM with 0 voltage at 1200 RPM for a 60 Hz machine. For a 50 Hz machine, synchronous speed will be approximately 1500 RPM. At ⅓ slip (i.e. 500 RPM slip), the rotor produces rated voltage, which corresponds to 1000 RPM and 2000 RPM with 0 voltage at 1500 RPM for a 50 Hz machine.
The consequence of this selection, however, is that at zero speed the rotor voltage would be three times the nominal voltage value (e.g. 2070V for a 690V power converter). Such a high voltage level at the rotor can severely damage the turbine electrical system. As such, it is imperative with a DFIG machine that the generator stator be de-energized when outside of its normal operating range.
Accordingly, an improved system and method for controlling a DFIG of a wind turbine that operates the generator stator at a reduced voltage (flux) to increase the speed range to allow more energy capture at low speed would be advantageous.